The present invention relates to an electric discharge machining (EDM) apparatus for and method of machining a workpiece submerged in a machining solution. More specifically, the invention relates to an apparatus for and method of optimizing machining of the workpiece.
In a conventional electric discharge machining apparatus, a workpiece is machined by applying a discharge current across an electrode and a workpiece. Typically, the electrode is made of a graphite material formed in a given shape or contour. The workpiece is machined to have a configuration which conforms with the contour of the electrode. Such a conventional EDM apparatus is shown in FIG. 1. As shown in the FIGURE, an electrode 10 is driven in a vertical direction (Z-axis direction) by a selection and drive circuit composed of elements 26, 28, and 30. Such an apparatus is widely known as a die-sinking EDM apparatus, which is extremely effective for manufacturing dies that have complicated constructions. Die-sinking EDM apparatuses have been previously disclosed in, for example, Japanese published patent application no. 3594/1966, and U.S. Pat. No. 4,400,606, both of which are herein incorporated by reference.
In operation, the EDM apparatus shown in FIG. 1 uses electrical discharge to machine the workpiece 14. Further, the electrode 10 is responsive to a feedback voltage that causes the electrode 10 to move upward or downward in accordance with its value. During this motion, an electric voltage V.sub.g is produced by a machining electric source 18, which is made up of a DC source 18a, a switching element 18b, a current limiting resistor 18c, and a pulse generator 18d. A periodic output from the electric source 18 creates a potential difference between the electrode 10 and the workpiece 14. The gap 20 formed between the electrode 10 and the workpiece 14 is known as the "interelectrode gap," and the voltage drop across that gap is represented in the FIGURE as V.sub.g. The workpiece 14 and the electrode 10 are submerged in a tank 12 filled with machining solution 16, as is the inter-electrode gap 20. When the voltage across the gap V.sub.g reaches a predetermined level, an electric discharge or arc is formed across the inter-electrode gap 20. As a result, the arc passes from the electrode 10 and terminates on the workpiece 14, creating a high temperature explosion at the workpiece, thus causing the workpiece surface to decompose. Typically, the surface is melted and dispersed as re-solidified chips that are retained in the machining gap 20. Due to a pumping action of the electrode 10 caused by a periodic up-and-down "jump" of the electrode, the machining solution washes most of the chips out of the gap 20.
The voltage V.sub.g is also supplied to amplifier 22 and is used as a feedback signal V.sub.s. This signal V.sub.s is subtracted from the reference voltage V.sub.r and the resulting signal is output from amplifier 24 to control the position of the electrode 10.
Although the above-described EDM apparatus is advantageous in that a workpiece can be machined without being restricted by the hardness of the work material and the shape that is to be machined, the EDM apparatus cannot operate at an optimal machining or removal rate when it is required to machine workpieces having high-tolerance corners and the like.
As shown in FIG. 3a, an electrode having a high-tolerance corner contour experiences its largest discharge current density in the corner region. Typically, as mentioned above, the electrode is made of a graphite material. Such material has low heat dissipation characteristics. As a result, the corner region of the electrode reaches temperatures in excess of 800.degree. C. At this temperature, pyrographite (carbon) material begins to adhere to the corner surface, as shown in FIG. 3b. As the buildup increases, the deposits begin to deform the shape of the original contour of the electrode, as shown in FIG. 3c. At the corner region, the pyrographite deposits form what is known as an "icicle" which begins to deform the contour of the workpiece during the machining process, resulting in an overcut of the contour, as shown in FIG. 3d. Typically, the pyrographite forms from the hydrocarbon in the dielectric machining solution found in the gap 20 between the electrode 10 and the workpiece 14. Where the workpiece is iron or steel, the icicle may be formed of the sludge found in the inter-electrode gap 20 instead of the carbon.
In order to accurately machine the workpiece, a visual inspection of the electrode 10 must be made periodically by an operator. As a result, the machining rate of the workpiece is reduced. The periodic interruption is significant in that only 20 minutes of operation is enough to generate an icicle having a significant buildup such that the accuracy of the machining contour will be affected.